The present invention relates to a process for producing isocyanate compounds which comprises producing a urethane compound from a formamide compound, or an amine compound, and dimethyl carbonate used as starting materials, followed by thermal decomposition thereof.
Isocyanate compounds, particularly those which have two isocyanate groups in the molecule, are useful as a starting material for polyurethane, polyurea, etc.
Isocyanate compounds are presently produced commercially by the reaction of a primary amine compound with phosgene. The phosgene process currently in use has difficulties in the handling of highly poisonous phosgene, disposal of hydrochloric acid, corrosion of equipment, etc., so that an industrially feasible production process which can replace it is eagerly desired. In recent years, with such a situation as the background, various processes which use no poisonous phosgene have been proposed.
Processes for producing an isocyanate compound which use no phosgene may be divided, when classified according to the principal starting materials, into those which use a formamide compound, amine compound and nitro compound, respectively. When classified according to the reaction steps, they may be divided into a process wherein an isocyanate compound is directly obtained by one-step reaction and a process for producing an isocyanate by two-step reaction wherein a urethane compound is produced first and then it is thermally decomposed.
The processes which use a formamide compound as the principal starting material include processes wherein isocyanates are directly obtained by dehydrogenation. U.S. Pat. No. 3,960,914 discloses a process which comprises dehydrogenating a formamide compound in a nonaqueous solvent with the aid of a catalyst of a platinum group element such as Pd, but the process has drawbacks of a low reaction rate and a selectivity toward isocyanate compounds of as low as 30%. U.S. Pat. No. 4,207,251 discloses a process comprising subjecting a formamide compound to a gas phase oxidative dehydrogenation for a specified contact time in the presence of Ag-containing catalyst, but the process is of little practical utility, the yield of isocyanate compounds being as low as 21% according to the working examples shown therein. U.S. Pat. No. 3,277,140 discloses a process comprising reacting a formamide compound with halogen such as bromine in the presence of a heterocyclic nitrogen compound, but the process gives an insufficient yield of isocyanate compound and consumes 4 moles of the heterocyclic compound as its halide per 1 mole of the isocyanate compound formed, and is hence economically unfavorable.
On the other hand, U.S. Pat. No. 4,661,217 discloses a process comprising oxidizing a formamide compound on a graphite electrode with NaBr used as the supporting electrolyte in an alcohol solvent to prepare a corresponding urethane compound. Since the urethane compound thus obtained can be converted into an isocyanate compound by thermal decomposition, there is suggested a possibility that a non-phosgene process which uses a formamide compound as a starting material can be developed. However, upon investigation of the urethane synthesis step of said process, it has been revealed that though a relatively high urethane yield is obtained a severe deterioration of the electrode takes place, so that the process has a difficulty in being put into practice in a commercial scale apparatus.
On the other hand, processes are known which produce urethane compounds from amine compounds and dimethyl carbonate (U.S. Pat. No. 3,763,217, E.P. No. 48,371, U.S. Pat. No. 4,395,565). These processes comprise reacting an amine compound with dimethyl carbonate in the presence of a Lewis acid catalyst, lead-, titanium- or zirconium-based catalyst, alkali catalyst, etc. According to the working examples shown therein, however, the processes generally have drawbacks of giving a low rate of reaction and being liable to form methylated compounds as by-products, leading to a low urethane yield. An improved process thereof has also been proposed, noticing the fact that specific amine compounds undergo the methylation of side reaction with difficulty, which comprises using dimethyl carbonate with a water content of less than 0.2% and adding the amine compound and sodium methylate catalyst thereto continuously or intermittently (PCT WO 88/05430).
The object of the present invention is to provide a process for producing an isocyanate compound economically and efficiently by using a formamide compound or an amine compound as a principal starting material and without using poisonous phosgene.